This invention involves the use of an image dye-forming coupler which releases a dye or dye precursor as the coupling off group. These are so-called "High Dye-Yield" (HDY) couplers because they form two molecules of dye for each molecule of coupler which is consumed.
Useful high dye-yield couplers have been disclosed by Mooberry and Singer in U.S. Pat. No. 4,840,884. (The '884 patent) Such couplers react with oxidized color developer to form one dye and in doing so release a precursor of a second dye having a neutral dye chromophore. In accordance with the patent, the new couplers described therein enable lower concentrations of silver halide in the photographic element without lowering image quality.
Among the desirable features of photographic elements are high speed (good low-light sensitivity) and low fog (non-imagewise development). Silver halide emulsions are prepared to have the most advantageous combination of high photographic speed and low fog development. This is done by design using for example, iodide content and distribution, grain size and morphology and by the degree of finishing in which the type or level of sensitization and additives which decrease fog formation are varied. Thus, there are various ways of controlling fog each having its own effect on speed. At the most advantageous speed/fog position the emulsion can be used most efficiently for imaging. However, at this speed/fog position, the emulsion may not give sufficient developed silver and so may not give sufficient dye formation (Dmax and density gradient) when coated with an imaging coupler, to make it useful in film embodiments.
In color negative processing in accordance with the Kodak Flexicolor C-41 process, the time of development (tod) is 3.25 minutes. As the tod is varied from 3.25 to 4.5 minutes both speed and fog will increase for an element given a step wedge exposure. If the emulsion is coated with an imaging coupler such that there is sufficient development at each tod to reach maximum dye density (Dmax), then, as tod is increased, the film contrast will increase along with the fog (read out as D.sub.min) until or unless the D.sub.min rises rapidly enough to suppress the contrast.
The region where the contrast and D.sub.min do not change substantially with changes in development time (around the 3.25 minutes used for conventional photographic elements) is the region of maximum efficiency for that coupler/silver combination.
High speed emulsions with high iodide contents are provided with addenda during finishing in a manner well-known in the art in order to sensitize the grains to obtain the desired high speed and contrast. This often leads to relatively high D.sub.min values. Emulsions finished to have lower D.sub.min values typically have lower contrast but may have adequate speed. High-speed, high-iodide emulsions are, generally, low in developability.
High iodide content emulsions show lower contrast and often, when this deficiency is combined with inefficiencies in dye formation and dye covering power, need to be coated at higher levels than would lower iodide level emulsions, in order to get the desired contrast. This, of course, interferes with the properties of the element.
It is a problem to be solved to provide an emulsion that is associated with a particular type coupler and that, when finished to have low D.sub.min and contrast, gives a sensitometric response that is improved over that of a more developable emulsion having a conventional coupler associated therewith.